1. Field Of The Invention
This invention relates to rotary, positive displacement blowers of the backflow type. More specifically, the present invention relates to reducing noise and/or improving efficiency of a Roots-type blower employed as a supercharger for an internal combusion engine.
2. Description Of The Prior Art
Rotary blowers of the Roots-type have long been characterized by noisy and/or inefficient operation. Attempts to decrease the source of the noise have generally decreased efficiency. The blower noise may be roughly classified into two groups: solid-borne noise caused by rotation of timing gears and rotor shaft bearings subjected to fluctuating loads, and fluid-borne noise caused by fluid flow characteristics such as rapid changes in fluid velocity and pressure. Rapid fluctuations in fluid flow and pressure also contribute to solid-borne noise.
As is well known, Roots-type blowers are similar to gear-type pumps in that both employ toothed or lobed rotors meshingly disposed in transversely overlapping cylindrical chambers and in that both transfer volumes of fluid from an inlet port to an outlet port via spaces between unmeshed teeth or lobes of each rotor without mechanical compression of the fluid. In both the Roots and gear devices, the top lands and ends of the unmeshed teeth or lobes of each rotor are closely spaced from the inner surfaces of the cylindrical chamber to effect a sealing cooperation therebetween. Since gear pumps are used almost exclusively to pump or transfer volumes of lubricious fluids, such as oil, the meshing teeth therein may contact to form a seal between the inlet and outlet ports. On the other hand, since Roots-type blowers are used almost exclusively to pump or transfer volumes of nonlubricious fluid, such as air, timing gears are used to maintain the meshing lobes in closely spaced, non-contacting relation to form the seal between the inlet and outlet ports.
This sealing arrangement between the meshing lobes, and between the lobes and cylindrical chamber surfaces makes a Roots-type blower substantially more prone to internal leakage than a gear pump. The liquid of a gear pump is substantially more viscous than the air of a Roots-type blower; therefore, oil is more leak-resistant. At any given time, a gear pump has several teeth per rotor in sealing relation with the cylindrical chamber surfaces which form a very effective labyrinth seal, whereas a Roots-type blower often has only one lobe per rotor in such sealing relation. Accordingly, Roots-type blowers are prone to internal leakage. The leakage, as a precentage of total displacement, increases with increasing boost pressure or pressure ratio and increases with decreasing speed of the rotors.
As previously mentioned, the transfer volumes of air trapped between the adjacent unmeshed lobes of each rotor are not mechanically compressed. Air, of course, is a compressible fluid. Accordingly, if the boost or outlet port air pressure is greater than the air pressure in the transfer volumes, outlet port air rushes or backflows into the transfer volumes as they move into direct communication with the outlet port with resultant rapid fluctuations in fluid volocity and pressure. Such fluctuations, due to backflow, are known major sources of airborne noise. In general, the noise increases with increasing pressure ratio and rotor speed.
Other major sources of airborne noise are cyclic variations in volumetric displacement of the blower due to meshing geometry of the lobes, and outlet air which is abruptly trapped between the remeshing lobes and abruptly returned to the inlet port. When a Roots-type blower is employed as a supercharger to boost the air or air/fuel charge of an internal combustion engine in a land vehicle, such as a passenger car, the blower is required to operate over wide speed and pressure ranges; for example, speed ranges of 2,000 to 16,000 RPM and pressure ratios of 1:1 to 1:1.8 are not uncommon. Prior art efforts to cost-effectively reduce or eliminate airborne noise from Roots-type blowers in such supercharger applications have, at best, met with limited success. In general, the efforts have successfully reduced airborne noise only for limited operating conditions of the blower, i.e., for specific boost pressure and rotor speed combinations. For example, a concept may effectively reduce airborne noise by reducing rapid fluctuations in fluid velocity and pressure at a high rotor speed and a high boost pressure; however, the concept is often totally ineffective at low rotor speed and high boost pressure. Further, in many cases, the efforts have increased internal leakage of the blower and, thereby, have decreased volumetric efficiency of the blower, have decreased energy efficiency, have undesirably increased the temperature of the boosted air, and have undesirably required an increase in blower size and/or speed.
U.S. Pat. No. 2,014,932 to Hallett addresses the problem of airborne noise; therein Hallett teaches that non-uniform displacement, due to meshing geometry, is reduced by employing helical twist lobes in lieu of straight lobes. Hallett asserts that helical lobed rotors, each having three lobes circumferentially spaced 120.degree. apart with a 60.degree. helical twist, best effects a compromise between the requirements of maximum displacement for a blower of given dimensions and a maximum frequency of pulsations of lesser magnitude. Theoretically, such helically twisted lobes would provide uniform displacement were it not for cyclic backflow and air trapped between the remeshing lobes.
Hallett also addresses the backflow problem and proposes reducing the initial rate of backflow to reduce the instantaneous magnitude of the backflow pulses. This is done by mismatched or rectangular-shaped inlet and output ports each having two sides parallel to the rotor axes and, therefore, skewed relative to the traversing top lands of the helical lobes. The parallel sides of the ports are positioned such that the cylindrical surface of each rotor chamber is a 180.degree. arc. With this lobe-port configuration, the lead lobe of each transfer volume traverses its associated outlet port boundary (i.e., the parallel sides) just as the trailing lobe of the transfer volume moves into sealing relation with the cylindrical wall surface; such an arrangement maximizes the time the trailing lobe is exposed to boosted or increased differential pressure and, thereby, maximizes the time for and rate of leakage across the trailing lobes.
Several other prior art patents also address the backflow problem by preflowing outlet port air into the transfer volumes before the top lands of the leading lobe of each transfer volume traverses the outer boundary of the outlet port. In some of these patents, as disclosed in U.S. Pat. No. 8,121,529 to Hubrich, preflow is provided by passages through the housing's cylindrical walls which sealingly cooperate with the top lands of the lobes. In U.S. Pat. No. 4,215,977 to Weatherston, preflow is provided in a manner similar to that of Hubrich. In a second embodiment of Wheatherston, preflow is provided by accurate channels or slots formed in the inner surfaces of the cylindrical walls which sealingly cooperate with the top lands of the lobes. The preflow arrangements of Hubrich and Weatherston, as with the backflow arrangement of Hallett, expose the trailing lobes of each transfer volume to boosted or increased pressure differential just as the trailing lobes move into sealing cooperation with the cylindrical wall surfaces and thereby undesirably maximize the time for and rate of leakage across the trailing lobes.